1. Field of the Invention
The invention relates to high-speed phototypesetters and particularly to those in which at least one dimension of movement for type selection is determined by movement of the illuminating beam.
2. Relation to the Prior Art
The old time typesetter selected pieces of type and assembled them in a type carrier which was then secured to a printing press, inked and used in printing. Phototypesetting today is more appropriately called photocomposition since the purpose is to provide a photographic image to serve as the basis for making printing plates. This photographic image must have all the desired font styles, sizes, spacings etc. that are to appear in the final print or at least in correct proportion where the final print is to be enlarged or reduced. Today all the photocomposing information is readily fed into a data processing system which can organize it and direct a phototypesetter at high speeds. Transparent type fonts have been arranged on a rotating drum with stationary flash lamps inside the drum. When the selected character came into alignment with the optical system, a flash lamp behind it would be triggered. The speed of such a system is limited by the drum cycle period. Higher speeds have been obtained using cathode ray tube phototypesetters. Cathode ray tube phototypesetters are expensive to manufacture. The cathode ray tube system permits the speed and precision of electronic scanning. The other systems providing the necessary flexibility require mechanical motion with its inherent problems of inertia. U.S. Pat. No. 2,600,168 to Klyce discloses a system similar in many details to the present one. The system does not use a focused image. Instead it uses a "shadowgraph". The shadowgraph technique relies on illumination from a point light source. The two biggest drawbacks to Klyce are the difficulty of providing sufficient illumination from a point source and diffraction problems. Klyce uses sets of orthogonally scanning mirrors to scan a font with the beam traversing each mirror twice so that the scanning motion is cancelled on return. Klyce separates the illuminating and return beams by beam splitters and uses further scanning mirrors to expose his photocomposing surface. Using a point light source and a shadowgraph image, the point light source must be very small and the character must be large. If the character is not very large, diffraction makes any shadowgraph image unusable. The fact that Klyce relies on a shadowgraph image is apparent from his use of a point light source 15 and that the plane of film 44 is located well beyond any focused image plane for characters 10.